An illumination light source comprising a light emitting device (hereinafter referred to as white LED) which emits white light by wavelength conversion of a phosphor by means of a light emitting diode (hereinafter referred to as LED) emitting light having a wavelength of 460 nm (blue), 405 nm (violet) or 380 nm (ultraviolet) encapsulated with an epoxy type transparent resin in a light-transmitting manner has a problem in poor durability since the transparent resin will deteriorate by light and heat of the white LED.
On the other hand, a fluoropolymer having a fluorinated alicyclic structure in its main chain has low refractive index properties and low surface energy properties, is excellent in transparency, light resistance (especially durability against a short wavelength light), chemical resistance, etc., and can be dissolved in a specific solvent. Therefore, a method of applying the fluoropolymer to an adhesive or coating agent to form a coating film having the above properties in various fields has been developed (see Patent Documents 1 and 2). Use of a coating film formed from a coating agent as disclosed in Patent Document 2 as a transparent resin for encapsulating a white LED in a light-transmitting manner has also been proposed (see Patent Document 3).
However, in the coating agent as disclosed in Patent Document 3, the concentration of the fluoropolymer is at a level of 25 mass % at the highest, whereby a thickness (at least 100 μm) required for encapsulating LED in a light-transmitting manner will not be obtained. It is possible to increase the thickness by applying the coating agent a plurality of times, but cracks are likely to occur in the coating film by recoating, thus lowering the yield.
Further, a process has been proposed to produce a molded product having high transparency to ultraviolet rays and excellent in transparency by bulk polymerization of a fluoropolymer such as perfluoro(2,2-dimethyl-1,3-dioxole) or perfluoro(butenyl vinyl ether)by means of a polymerization initiator comprising a fluorinated organic peroxide (see Patent Document 4). However, a polymer of the fluoromonomer is a thermoplastic linear monomer and has not necessarily sufficient heat resistance. Further, the fluoromonomer has a low boiling point and if the bulk polymerization is carried out in an open system, the fluoromonomer will volatilize, whereby it will be difficult to control the volume of the molded product.
Patent Document 1: JP-A-2-84456
Patent Document 2: JP-A-2-129254
Patent Document 3: JP-A-2003-8073
Patent Document 4: JP-A-2000-1511